Orthogonal Frequency Division Multiplexing (OFDM)
OFDM (Orthogonal Frequency Division Multiplexing) is a digital multicarrier modulation technique in which a data stream is transmitted in parallel over many narrowband, orthogonal subcarriers. It was developed to reduce interference and to improve robustness against multipath propagation in wireless systems. The parallel transmission increases resilience to frequency-selective fading and simplifies equalization in the receiver.
Key Technical Characteristics
Orthogonality
Subcarriers are spaced so that they overlap in the frequency domain without mutual interference, maximizing spectral efficiency.Subcarriers and Modulation
A 20 MHz OFDM channel typically uses dozens to hundreds of subcarriers. Each subcarrier is modulated using QPSK, 16-QAM, or 64-QAM, depending on the channel conditions, to optimize throughput and reliability.IFFT/FFT Processing
OFDM signals are generated using an inverse fast Fourier transform (IFFT) and demodulated with an FFT. This enables efficient implementation on digital signal processors (DSPs) or other hardware platforms.Guard Interval / Cyclic Prefix
To mitigate inter-symbol interference (ISI) caused by multipath propagation, a cyclic prefix is inserted between OFDM symbols.
Distinction
FDM vs. OFDM
Classical Frequency Division Multiplexing (FDM) uses separated carriers with guard bands. OFDM relies on tightly spaced, orthogonal subcarriers without guard bands, improving bandwidth efficiency.
OFDM vs. OFDMA
OFDMA is the multi-user extension of OFDM. It enables simultaneous communication with multiple users by dynamically assigning groups of subcarriers (Resource Units) to different users.
Application Examples
Wi-Fi: Used in Wi-Fi 4 (802.11n) and Wi-Fi 5 (802.11ac) for PHY-layer modulation
Mobile Networks: LTE downlink uses OFDM; uplink uses SC-FDMA
Digital Broadcasting: DVB-T, DVB-T2 for terrestrial television
Powerline Communication: Employed in technologies such as HomePlug
5G NR: Uses scalable OFDM with variable subcarrier spacing across frequency bands to support applications such as enhanced Mobile Broadband (eMBB) and Ultra-Reliable Low-Latency Communication (URLLC)